Multi-exposure techniques are widely used for improving the dynamic range of video cameras. The dynamic range of a video signal generated by an image sensor is limited by its noise floor on the one hand and the saturation voltage on the other hand. For the lowest level in a typical scene, the signal to noise ratio (SNR) needs to be at least about 40 dB to have an acceptable quality. Therefore the total dynamic range should be 100 dB.
To achieve this, there is known the double exposure solution, where two pictures are taken shortly after one another: one with a short exposure time and one with a long exposure time. Combining the two can give a good SNR for the long exposed image in the dark part and can avoid saturation in the bright part for the short exposed image.
However, in artificial light sources, particularly fluorescents, light is modulated at twice the local mains frequency. If the integration time of the sensor is not a multiple of the period of the fluorescent light source, the amount of integrated light varies per field (frame), which creates a problem of flickers and changing colors.
The frequency of fluorescent light flicker is either 100 Hz or 120 Hz, according to national standards, and can vary up to 2%. To cope with this problem, there is usually provided either a manual switch or a flicker detection mechanism that sets the sensor integration time to an integer number of the fluorescence period (e.g. n/100 s or n/120 s respectively, depending on the national mains frequency, n=1,2,3, . . . ).
This special operation is a valid solution for single exposure time sensors, but leads to problems in connection with multiple exposure time sensors. If, for instance, the longer exposure time is chosen as 1/100 s, the shorter exposure (integration) time will be several times shorter than that of the longer exposure time (the exact relation depends on their ratio R) and will not be adequate for the operation under fluorescence light conditions.
International Patent Application WO 2007/038977 A1 discusses an image pickup apparatus comprising an image pickup arrangement for forming a plurality of image signals having different exposure conditions, a combining arrangement for combining said plurality of image signals to form a combined image signal having an extended dynamic range, further comprising display and/or a recording arrangement for displaying and/or recording said combined image signal, further comprising a function module correcting at least one of the image signals in order to achieve a smooth transition between the image signals at a transition point.